They probably do it in the processor, and the board was never updated due to retooling costs for the production line.

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Originally posted by mbroyles Not knowing what the DAC's current drive capability is, how stable and high freq trash free it is, I would be reluctant to chop the first 2 opamps out of the circuit.

Which begs the question: "Why don't you know?"

You could have consulted the DAC IC's Datasheet which is readily available on the net and you would have found out that the DAC will drive an 600R AC per phase and a 1K DC per Phase load, has 3.5mA rated output current or theoretically 8.2V Peak into 1K2 Balanced AC Load.

My final recommended output confioguration would be to use a 1.2K Load Resistor per phase in parallel with 1.5nF capacitance and a 4.7mH Inductor in series to the DAC Chip, per phase and DC coupled, take from each junction point a 2.2uF (or larger) coupling capacitor.

This circuit allows loads down to 10K per phase or 20k balanced, retains the balanced output and allows 1nF per phase or 500pF balanced cable capacitance, in other words more than enough to accomodate most sensible cables and amplifiers, but prohibiting extreme loads (which a 600R:600R line output transformer may actually allow).

FWIW, at the stopband return of the Digital filter (assuming 96KHz operation) the passive filter will be > 40db down, for 44.1KHz operation there will still be 30db attenuation, sufficient for most competently designed amplifiers to handle okay.

Originally posted by Thmartin Yup, pretty sure. Let's say 99%.
Anyway I'll measure it before I turn the amps on.

Thierry,

Yes, please measure it. You'll find approximately 2.6 VDC on each of the four output pins on each chip. A simple low-pass filter like you described (I hope my reading of your description didn't get lost in the translation...I don't think so because your English seems fine) will not do anything to block or balance this DC level and it will be passed on to the following stage.....whatever that may be.

My conclusion was that it looks good on paper but was bad in reality. The problem is that there seems to be a lot of RF floating around inside DCX and the L part of the filter will pick it up like antenna.

I can post measurements later when I get home. I looked at the spectrum of 10Hz-40kHz and the LC type filter created a lot more dirt that raised above the noisefloor.

Thus I burried the idea of LC and now my setup will be

RC -> 6ch volume pot -> RC -> OPA2134 in simple gain mode -> output.

I ran the DCX with simple RC filtering and no analog stage a long time, some 6 months or so, but in the end I did not like the sound especially in top octaves. There is something wrong there. At first it gives a very positive impression but gets tiring and irritating after a month or two.

Originally posted by AndrewT This circuit blocks DC as well as providing balanced source impedances.

Surprising no one has picked up on this yet.

It is not surprising AT ALL. The circuit you cite is aimed at the exact opposite problem to the one faced wwhen directly interfacing the DCX2496 DAC's, that is it is meant to give a Pseudo-balanced output from a single ended source, while we need one that gives a SE source from a true balanced output.